This application claims the benefit of Korean Patent Application No. 1999-31487, filed on Jul. 31, 1999, under 35 U.S.C. xc2xa7 119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display device having a thin film transistor (TFT) and a method of manufacturing the same.
2. Description of Related Art
A typical liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in arrangement resulting from their thin and long shapes. The arrangement direction of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the arrangement direction of the liquid crystal molecules is changed, the arrangement of the liquid crystal molecules also changes. Since Incident light is refracted to the arrangement direction of the liquid crystal molecules due to the optical anisotropy of the arranged liquid crystal molecules image data can be displayed.
By now, an active matrix LCD that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is most attention-getting due to its high resolution and superiority in displaying moving video data.
FIG. 1 is a cross-sectional view illustrating a pixel of a conventional liquid crystal display device.
The liquid crystal display device 20 has a bottom substrate 2 and a top substrate 4 spaced apart from each other. The liquid crystal display device further includes a liquid crystal layer 10 is injected between the two opposite substrates 2 and 4. The top substrate 4 has a color filter to display colors, and the bottom substrate 2 has switching elements such as thin film transistors (TFTs) that applies electrical signals to the liquid crystal layer 10 to change the arrangement direction of the liquid crystal molecules of the liquid crystal layer 10. Each of the TFTs xe2x80x9cSxe2x80x9d has a gate electrode 30, a source electrode 32 and a drain electrode 34.
In detail, the top substrate 4 further includes a common electrode 12 covering the color filter layer 8. The common electrode 12 plays a role of the first electrode to supply a voltage to the liquid crystal layer 10. The bottom substrate 2 further includes a pixel electrode 14. The pixel electrode 14 is electrically connected with the drain electrode 34 of the TFT xe2x80x9cSxe2x80x9d. The pixel electrode 14 receives electrical signals from the thin film transistor xe2x80x9cSxe2x80x9d, and plays a role of the second electrode to supply voltage to the liquid crystal layer 10. A portion, on which the pixel electrodes 14 are formed, is defined as a pixel electrode portion xe2x80x9cPxe2x80x9d. In order to prevent leakage of the liquid crystal layer 10 disposed between the top substrate 4 and the bottom substrate 2, edge portions of the top substrate 4 and the bottom substrate 2 are sealed by a sealant 6.
Recently, as the display area of the liquid crystal display device becomes larger, the fabricating process of the bottom substrate 2 becomes complicated. That is to say, for a liquid crystal display device having over 12 inch display area, a step-and-repeat exposure technique is applied to fabricating the bottom substrate. The step-and-repeat exposure technique is to perform at least two exposing steps with the same patterned mask. The reason for the step and repeat technique to be applicable to the bottom substrate is that the patterns formed on the bottom substrate are repeats of the same form.
Referring to the FIGS. 2 and 3, a batch exposure technique and the step-and-repeat exposure technique are explained as follows. Referring to the FIG. 2 showing a patterning mask, in the batch exposure technique, a display area A, data pad portions D and E and gate pad portions B and C surrounding the display area xe2x80x9cAxe2x80x9d are formed at one time with the patterning mask.
The batch exposure technique is just applicable to the bottom substrate of the liquid crystal display device having a less than 10 inch-sized display area. Namely, in case of the bottom substrate of the liquid crystal display device having a larger than 10 inch-sized display area, the batch exposure technique is useless due to the diffraction of light incident from an exposure apparatus.
Referring to the FIG. 3 illustrating the step-and-repeat exposure technique, the display area is formed into a plurality of neighboring display exposure regions like A1, A2, . . . , A9, sequentially. Each of the display exposure regions has an identical image projected onto itself with a same display patterning mask.
By the same technique, the data pad portions are formed into a plurality of neighboring data exposure regions like D1, D2, D3 and E1, E2 E3 having an identical image, sequentially. And the gate pad portions are formed into a plurality of neighboring gate exposure regions like B1, B2, B3, C1, C2 and C3 having an identical image, sequentially.
The above-mentioned step-and-repeat exposure technique is more widely used than the batch exposure technique as an exposure method.
But, to fabricate the liquid crystal display device using the step-and-repeat exposure technique may give rise to a serious degradation of image quality at the display area. The reason is that the step-and-repeat exposure technique needs at least over 40 processes of photolithography. Comparing with the step-and-repeat exposure technique, the batch technique needs at least just 5 processes of photolithography. Thus, no matter how accurate exposure equipment and arrangement apparatus are used for the step-and-repeat exposure technique, it may give rise to misalignment between the exposure regions.
For example, as shown in FIG. 4, the display exposure regions A7 and A8, the display exposure region A7 includes a pixel electrode 71, a half of a data line 60, and a half of a data line 61. The display exposure region A8 includes a pixel electrode 72, a half of a data line 61, and a half of a data line 62. The display exposure regions A7 and A8 include the data line 61 in common, and are divided by an imaginary boundary line 50. That is to say, the display exposure regions A7 and A8 differ in an exposed order with the imaginary boundary line 50 centering on between the display exposure regions A7 and A8. That difference in the exposed order may bring out a difference in distances between the pixel electrodes 71 and 72 and the data lines 60, 61 and 62. Since the exposure equipment or the arrangement apparatus has an accuracy limitation of itself, misalignment between the exposure regions may occur. The misalignment may result in shift, rotation and distortion of the patterns, thereby causing defects such as disconnection of the wirings and differences in electrical properties between the exposure regions.
Namely, the distance between the pixel electrode 71 and the data line 60 is different from the distance between the pixel electrode 71 and the data line 61. And, the distance between the pixel electrode 72 and the data line 61 is different from the distance between the pixel electrode 72 and the data line 62. The pixel electrodes 71 and 72 are the pixel portions P1 and P2, respectively.
In other words, fabricating the thin film transistor by the step-and-repeat exposure technique, it may bring about spotted effects near the boundaries of the neighboring display exposure regions resulted from the sudden difference in the distance between the pixel electrodes and the data lines at each exposure region.
In case of manufacturing the large-sized liquid crystal display device using the step-and-repeat exposure technique, driving the liquid crystal display device by a dot inversion method, it brings about the difference in parasitic capacitance Cdp between the data line and the right and left pixel electrodes between exposure regions. The parasitic capacitance Cdp is the critical factor directly affecting the brightness of the display area. Thus, the difference in the parasitic capacitance Cdp brings about the difference in the brightness between the left pixel electrode and the right pixel electrode with the center boundary line differentiating the left pixel electrode and the right pixel electrode. Namely, the detectable difference in brightness, or the spotted effect occurs near the boundaries of the exposure regions.
It is therefore an object of the present invention to reduce the detectable difference in brightness resulting from the difference in distance between pixel electrodes and data lines at the boundaries of exposure regions for a liquid crystal display device having a larger display area.
For the above object, it is a preferred embodiment of the present invention, a method of manufacturing a liquid crystal display device, including depositing a first metal layer on a transparent substrate; patterning the metal layer to form a gate line, the gate line having a gate electrode portion; depositing sequentially an insulating layer, an amorphous silicon layer and a doped amorphous silicon layer on the exposed surface of the transparent substrate while covering the gate line; patterning the amorphous silicon layer and the doped amorphous silicon layer to form a semiconductor island; depositing a second metal layer on the exposed surface of the insulating layer while covering the semiconductor island; patterning the second metal layer to form a source electrode, a drain electrode, and a capacitor electrode, the drain electrode spaced apart from the source electrode; etching the doped amorphous silicon layer of the semiconductor island to from an active area; forming a passivation film over the whole surface of the substrate while covering the source electrode, the drain electrode and the capacitor electrode; depositing a transparent conductive material layer on the passivation film; applying a negative photoresist on the transparent conductive material layer; performing a back side exposure to form a first exposed portion of the negative photoresist; aligning a patterning mask with the negative photoresist; performing a front side exposure to form a second exposed portion of the negative photoresist, the second exposed portion overlapping the first exposed portion; baking the transparent conductive material layer; and patterning the transparent conductive material layer to form a pixel electrode.
The gate line further includes first and second light shielding portion, the gate electrode portion interposed the first and second light shielding portions, the first and second light shielding portions extended outward a direction perpendicular to the gate line. An overlapped portion that the first exposed portion overlaps the second exposed portion is about 2 xcexcm to about 4 xcexcm in width. A temperature of baking the transparent conductive material layer is about 100xc2x0 C. to about 150xc2x0 C.
In another aspect, a liquid crystal display device includes a display area including gate lines, data lines, and thin film transistors, the gate lines arranged in a direction, the data lines arranged a direction perpendicular to the gate lines, the thin film transistors arranged near cross points of the gate lines and the data lines; a gate pad portion having a plurality of gate pads, each of the plurality of the gate pads connected with the corresponding gate lines; a data pad portion having a plurality of data pads, each of the plurality of the data pads connected with the corresponding data lines; and a plurality of light shielding patterns arranged along and outside edges of the display area, the light shielding patterns preventing light from transmitting portions other than the display area and the gate and data pad portions.
The light shielding patterns are made of an opaque material. The light shielding patterns are selected from a group consisting of chromium (Cr), aluminum (Al), antimony (Sb), tungsten (W), tantalum (Ta), molybdenum (Mo) and amorphous silicon. The light shielding patterns includes two gate light shielding patterns and two data light shielding patterns, the two gate light shielding patterns arranged in a direction parallel to the data lines and spaced apart from each other with the display area therebetween, the two data light shielding patterns arranged in a direction parallel to the gate lines and spaced apart from each other with the display area therebetween. The light shielding patterns including a plurality of light shielding patterns, the plurality of the light shielding patterns spaced apart from each other, both end portions of each of the plurality of the light shielding patterns overlapping a portion of the gate lines or the data lines.
In another aspect, a method of fabricating an array substrate of a liquid crystal display device including a transparent substrate and a plurality of gate and data pads, the method includes forming a plurality of gate lines and a plurality of gate pads, the plurality of the gate lines arranged in a direction, each of the gate pads connecting with the corresponding gate line outside the display area by a step-and-repeat exposure technique with a front-side exposure; forming data light shielding patterns parallel to the gate lines between pre-positions of the data pads and the display area; forming a plurality of data lines and data pads, the plurality of the data lines arranged a direction perpendicular to the gate lines, each of the data pads connecting with the corresponding data line outside the display area by the step-and-repeat exposure technique with the front-side exposure; forming gate light shielding patterns parallel to the data lines between the gate pads and the display area; forming a thin film transistor arranged near cross portion of the gate and data lines, the thin film transistor having a gate electrode, a source electrode and a drain electrode; depositing a transparent conductive layer and applying a negative-photoresist on the transparent substrate having the thin film transistor; forming a first exposed portion of the negative-photoresist by back-side exposure using the gate and data lines and the gate and data light shielding patterns as a mask; forming a second portion of the negative-photoresist using a step-and-repeat exposure by a front-side exposure; backing the transparent conductive layer; and etching the transparent conductive layer to form a pixel electrode.